Serial Digital Interface

The Serial Digital Interface (SDI), a serial digital interface, primarily for the transmission of uncompressed and unencrypted video data over coaxial cables or fiber optic cables. It comes mainly in the professional television studios and in the field of television stations for use.

The SDI interface is specified by the Society of Motion Picture and Television Engineers (SMPTE ) and represents a further development of the analog video standards such as PAL or NTSC method dar.

Bit rates

The SDI interface is divided into different groups with different bit rates: In the SD -SDI group which reflect, among other things, the standard resolutions of PAL or NTSC and most with 270 Mbit / s is operated. In the standard SMPTE 292M HD -SDI interface is described, which is since 1998 for use in the context of HDTV formats, and usually has a bit rate of 1.485 Gbit / s. An extension of HD -SDI, 3G -SDI, is operated with 2.97 Gbit / s and is defined in the standard SMPTE 424M.

The following are the main SDI interface standards are summarized. The number under video format is thereby the number of lines on which subsequent letter i stands for interlaced, as is common in PAL or NTSC, and p stands for progressive, imagery that is transmitted without interlacing.

Physical interfaces

The SDI interface is based on electrically coaxial cable with an impedance of 75 Ω and used to connect BNC connectors. The cable and connector types used were taken from the previous analog video signal standard. The maximum cable length for SD -SDI and used in accordance with quality cables up to 300 m, with HD -SDI, the maximum cable length is 100 m. The voltage is 800 mV ( ± 10%).

Further, particularly in the HD-SDI, and an optical transmission is used by means of light waveguides. The parameters are defined in the SMPTE 292M, which specifies, among other things, an optical wavelength of 1310 nm ( ± 40 nm), SC connectors and fiber optic cable according to IEC 60793-2.

The data is independent of the media as NRZI encoded data stream in order to enable the receiver, the clock recovery of the pixel clock. In addition, the data will be made statistically direct component using scrambler in order to lead as the SDI stream for galvanic isolation via pulse transformers can.

Data format

In SMPTE 259M, which is equivalent to the ITU -R BT.656, the basic frame structure of an uncompressed digital video signal will be described. The format and temporal orientation is similar to the analog video standards. Certain analog specifications, as necessary for the horizontal and vertical synchronization control pulses are shown here through a special digital codes, the SAV ( Start of Active Video engl. ) and EAV ( End of Active Video engl. ).

SD -SDI, the word width of the transmitted luminance data (Y ) and color information (Cb is the blue color information and Cr for the red color information ) is typically 10 bits. In HD -SDI, the Y and Cb / Cr data are mapped to parallel 10 bits in a 20 bit wide word.

The image information comprises at SD -SDI following chronology on:

And HD -SDI, always a Y is summarized with a parallel C- value:

The image information is transmitted in the YCbCr color model with a sampling grid of 4:2:2 and contain only the value range 4-1019 ( 0x004 to 0x3FB in hexadecimal notation ). The values ​​0 to 3, and from 1020 to 1023 ( 0x3FC to 0x3FF ) are reserved within the SDI interface for signaling and control words. Among other things, this is signaled by control words the start or end of a video line.

Extensions

In addition to the uncompressed video signal is primarily transferred, it is possible to transmit extra signals, control data, or control data SDI. For this purpose, the time ranges outside the visible video frame which are used in the underlying analog video standards for synchronization pulses used for the transmission of special data packets.

The standard SMPTE 291M specifies how this additional data, known in English as ancillary data can be inserted into the SDI signal. Devices, which make this insertion or embedding are also referred to as the embedder and the counterpart as Deembedder. For example, the following data can be additionally inserted to the video data:

  • EDH packets for error detection (English Error Detection and Handling ) according to the standard SMPTE RP168.
  • Digital audio data according to the standard AES - third It can be inserted up to 16 uncompressed audio channels in both SD -SDI and HD -SDI, SMPTE 272M and its method in SMPTE 299M are fixed.
  • Time code timing.

The advantage of embedded auxiliary data is that no separate ways for video signals and the corresponding audio signals and control signals are required as with analog transmission in the infrastructure of a television studio. Contains the image signal, aligned according to time correctly, its associated up to 16 audio streams and can be performed as a signal sequence with a constant delay time, for example, cross rails.

SDTI

In addition to the transport of uncompressed video data is in the SDI interface is also the possibility to use the normally visible area of ​​the video data for the transport of arbitrary data. Application lie in being able to transfer such as Ethernet interfaces or digitally compressed video data to MPEG or H.264 on existing SDI infrastructure. In order not to interfere with the picture display, the method in SDTI ( Serial Data Transport Interface Data Sheet ) is fixed. For SD -SDI SMPTE 305M specifies the details for the HD -SDI SMPTE- 348m.

Literature sources and standards

In addition to those listed in Article SMPTE standards:

  • Charles Poynton: Digital Video and HDTV Algorithms and Interfaces. Morgan Kaufmann Publishers, San Francisco, 2003, ISBN 1-55860-792-7.
  • Video Interface
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